The vertebrae are bones that make up the spinal column, which surrounds and protects the spinal cord. Nerves from the spinal cord exit the spinal column between each vertebra. Intervertebral discs are soft tissues positioned between each vertebrae. The discs act as cushions between the vertebrae by absorbing energy while the spinal column flexes, extends, and twists. The disc allows for movements of the vertebrae and lets people bend and rotate their neck and back. The type and degree of motion varies between the different levels of the spine: cervical (neck), thoracic (chest) or lumbar (low back). The cervical spine permits movement in all directions. The thoracic spine protects the heart and lungs and is more rigid than the cervical spine due to rib presence. The lumbar spine permits primarily forward and backward bending movements, flexion and extension.
Spinal fusion is a surgical technique, typically involving the use of bone graft, that causes two opposing bony surfaces to grow together (arthrodesis). In spinal fusion, one or more of the vertebrae of the spine are fused to prevent any motion therebetween. At each level in the spine, there is a disc space in the front and paired facet joints in the back. Working together, these structures define a motion segment and permit multiple degrees of motion. Two vertebral segments need to be fused to stop the motion at one segment. Generally, spinal fusion surgery involves adding bone graft to an area of the spine to prompt a biological bone growth response. The growth of bone between the two vertebrae is facilitated by the bone graft and effectively stops motion at that segment.
There are many potential reasons for spinal fusion. Example reasons include: treatment of a fractured vertebra; correction of deformity; elimination of pain from painful motion; treatment of instability; and treatment of cervical disc herniations. While not all spinal fractures need surgery, some fractures, including those associated with spinal cord or nerve injury, generally require fusion as part of the surgical treatment. In spondylolisthesis, a hairline fracture allows vertebrae to slip forward on top of each other. This condition may be treated by fusion surgery. Certain types of spinal deformity, such as scoliosis, are commonly treated with spinal fusion. Another condition treated with fusion surgery is actual or potential instability (or abnormal or excessive motion between two or more vertebrae). Cervical disc herniations requiring surgery often call for fusion as well as removal of the herniated disc (discectomy). With this procedure, the disc is removed through an incision in the front of the neck (anteriorly) and a small piece of bone is inserted in place of the disc. Although disc removal is commonly combined with fusion in the cervical spine, this is not generally true in the lumbar spine. Further, spinal fusion may be called for in the treatment of a painful spinal condition without clear instability.
It is important to note that spinal fusion surgery does not involve merely the knitting of a bone around a spinal fracture, although a spinal fracture may be the impetus for performing the spinal fusion. While a side-consequence of the spinal fusion surgery or of the natural biological response to a fracture may involve knitting of the fracture, spinal fusion involves stimulating nonanatomic or heterotopic bone growth between two vertebra to fuse the vertebra together.
Many surgical approaches and methods involving placement of a bone graft between the vertebrae can be used to fuse the spine. The spine may be approached and the graft placed either from the back (posterior approach), from the front (anterior approach) or by a combination of both approaches. The type and location of the incision for access to the spinal region depends on the area needing treatment. The lower spinal vertebrae are repaired through an incision directly over the spine (posterior lumbar approach). The upper spinal vertebrae are repaired through an incision in the back or side of the neck (cervical spine). The middle spinal vertebrae are repaired through an incision made in the chest and abdomen (anterior thoracic spine). The abnormal or injured vertebrae are repaired and stabilized with bone grafts, metal rods, other instrumentation, or a combination of the above.
There are several types of spinal fusion, including:                Posterolateral gutter fusion        Posterior lumbar interbody fusion (PLIF)        Anterior lumbar interbody fusion (ALIF)        Anterior/posterior spinal fusion        Cervical fusion        Thoracic fusion        Interlaminar fusion        
Posterolateral gutter fusion involves placing bone graft in the posterolateral portion of the spine (a region just outside the spine). The surgical approach to the spine is from the back through a midline incision that is approximately three inches to six inches long. Typically, bone graft is obtained from the pelvis (the iliac crest) and the harvested bone graft is laid out in the posterolateral portion of the spine. The back muscles that attach to the transverse processes are elevated to support the bone graft. The back muscles are replaced over the bone graft to create tension to hold the bone graft in place. After surgery, the body attempts to heal itself by growing bone. The growth of bone by the body grows the harvested bone graft and adheres the graft to the transverse processes. At this point, spinal fusion is achieved and motion at that segment is stopped.
Posterior lumbar interbody fusion (PLIF) involves adding bone graft to an area of the spine to set up a biological response that causes bone to grow between the two vertebrae and stop the motion at that segment. PLIF achieves spinal fusion by inserting bone graft directly into the disc space. The spine is approached through an incision (typically three to six inches long) in the midline of the back and the left and right erector spinae are stripped off the lamina on both sides and at multiple levels. After the spine is approached, the lamina is removed (laminectomy) to allow visualization of the nerve roots. The facet joints, which are directly over the nerve roots, are trimmed to make more space for the nerve roots. The nerve roots are retracted to one side and the disc space is cleaned of the disc material. A bone graft, or interbody cage with bone, or other instrumentation or implant, is inserted into the disc space and the bone grows from vertebral body to vertebral body.
Anterior lumbar interbody fusion (ALIF) is similar to the PLIF, except that in ALIF the disc space is fused by approaching the spine through the abdomen instead of through the back. In the ALIF approach, an incision (typically three to five inches) is made on the left side of the abdomen and the abdominal muscles are retracted to the side. The peritoneum can also be retracted to allow the surgeon access to the front of the spine. Some ALIF procedures are done using a minilaparotomy (one small incision) or with an endoscope (a scope that allows the surgery to be done through several one-inch incisions). Regardless of the specific procedure, the aorta and vena cava are moved aside, the disc material is removed and bone graft, or bone graft and anterior interbody cages, other implant, or instrumentation, is inserted.
Anterior/posterior lumbar fusion involves performing a lumbar interbody fusion and a posterolateral gutter fusion to fuse both the front and back of the spine. Fusing both the front and back provides a high degree of stability for the spine and a large surface area for the bone fusion to occur. The disc may be approached either as an ALIF or as a PLIF, and the back part of the spine is fused with a posterolateral gutter fusion.
The ultimate goal of fusion is to obtain a solid union between two or more vertebrae. This is done by causing non-anatomic growth of bone, most commonly around a bone graft, instrumentation including a bone graft, or other implant. Spine fusion involves causing bone to grow in a space where it does not normally grow, that is, in the space between two vertebra. Spine surgery instrumentation is sometimes used as an adjunct to obtain a solid fusion. This is particularly advantageous because causing non-anatomic or heterotopic growth of bone attempts to take advantage of the body's natural healing process of growing bone and yet use that process in a way that would not normally occur. Non-anatomic or heterotopic growth in this manner differs significantly from the mere knitting of a fractured bone and can be difficult to achieve. A common problem in spine fusion is that it is nonsuccessful and results in non-union. Instrumentation can decrease the likelihood of non-union by maintaining spinal stability while facilitating the process of fusion. The instrumentation can be used to bridge space created by the removal of a spinal element such as an intervertebral disc. Instrumentation is sometimes used to correct a deformity, but usually is solely used as an internal splint to hold the vertebrae together while the bone grafts heal. Such instrumentation may include titanium, titanium-alloy, stainless steel, or non-metallic devices for implantation into the spine. Instrumentation provides a permanent solution to spinal instability. Medical implants are available in many shapes and sizes. Typically these include rods, hooks, braided cable, plates, screws, and threaded interbody cages. Unfortunately, even with the use of instrumentation, non-union remains a common problem.
Regardless of whether instrumentation is used, bone or bone substitutes are used to prompt the vertebrae to fuse together. Traditionally, the surgical technique includes a grafting procedure utilizing autologous bone harvested from a separate site. In a typical procedure, bone chips from a patient's pelvic bone are transplanted, or grafted, to the spinal vertebrae to help fusion therebetween. Alternatively, allograft, bone harvested from a bone bank or other source, may be used. Similarly, synthetic and xenograft derived bone substitutes (calcium phosphate, hydroxylapatite, and/or other ceramic based bone substitutes) may be used.
Although the use of autologous bone for spinal fusion is common, harvesting bone graft from a patient's body has many disadvantages. Among other things, graft harvesting prolongs surgical time, increases blood loss, increases the risk of infection, and can be a source of chronic pain. Significantly, use of autologous bone does not always ensure successful fusion, even when used in combination with instrumentation. Inherent limitations in autogenous and allogeneic bone grafting have led to exploration of other technology, for example, using bone morphogenic protein (BMP) in spinal fusion. As an adjuvant to allograft or as a replacement for harvested autograft, BMP can improve spinal fusion.
Bone morphogenic protein (BMP) is in the bone's own matrix and has the ability to stimulate the body's own cells to produce more bone. Specifically, BMP can direct the repair and regeneration of bone in various parts of the skeleton. BMP can be beneficial to patients undergoing spinal fusion by eliminating the need for bone transplantation from the pelvis. BMP has been used to promote bone fusion by putting BMP on a mesh, gel, or other carrier and placing that carrier at the site where the bone fusion is desired.
To function as a suitable graft for spinal fusion—involving bridging bone defects or fusing facture lines and unstable motion segments—the graft must have three characteristics. It must provide a source of primitive osteoprogenitor cells that form osteoblasts and osteocytes (osteopromotion). The graft material must produce local growth factors to stimulate bone growth and vascularity in the area (osteoinduction). Lastly, it must act as a scaffold to bone ingrowth (osteoconduction).
BMP has a multifaceted osteoinductive role, acting as a chemotactic agent, a growth factor, and a differentiation factor. As a chemotactic factor, it initiates the recruitment of progenitor and stem cells toward the area of bone injury. As a growth factor, it stimulates both angiogenesis and the proliferation of stem cells from surrounding mesenchymal tissues. As a differentiation factor, it promotes maturation of stem cells into chondrocytes, osteoblasts, and osteocytes.
Thus far, growth and differentiation factors have been obtained in a variety of ways for application directly to a surgical site. These include: extraction of the factors from animal or human bone matrix, production of a single factor by cellular hosts by using recombinant technology, and direct delivery to cells at the site of desired bone formation of the DNA encoding for the factor.
Although BMPs are potent stimulators of bone formation, there are disadvantages to their use in enhancing bone healing. Receptors for the BMPs have been identified in many tissues, and the BMPs themselves are expressed in a large variety of tissues in specific temporal and spatial patterns. This suggests that BMPs may have effects on many tissues in addition to bone and their usefulness as therapeutic agents, particularly when administered systemically, may be limited. However, their usefulness is not only suspect when administered systemically, there are equally serious concerns regarding local administration of BMPs to a surgical site. It is difficult to control BMP's effect on bone growth and surrounding tissue. The consequences of BMP's effect on bone growth at spinal fusion sites is particularly concerning. Hypergrowth as a result of BMP application directly to a spinal fusion site may pinch nerves and result in not only pain but possibly paralysis. Consequently, it may not be desirable to apply BMP directly to a spinal fusion site.
HMG-CoA reductase inhibitors, or statins, have been known to promote BMP production. U.S. Pat. Nos. 6,080,779, 6,376,476, and 6,022,887 each disclose using HMG-CoA reductase inhibitors to promote bone formation systemically or at, for example, fracture sites. HMG-CoA reductase is the principal rate limiting enzyme involved in cellular cholesterol biosynthesis. The pathway is also responsible for the production of dolichol, ubiquinones, isopentenyl adenine and farnesol. HMG-CoA reductase converts 3-hydroxy-3-methyld-glutaryl CoA (HMG-CoA) to mevalonate. The '779, '476, and '887 patents do not contemplate use of HMG-CoA reductase inhibitors to promote non-anatomic or heterotopic growth of bone as, for example, in spinal fusion.